Multilayered magnetic free layer structure containing an ordered magnetic alloy first magnetic free layer for spin-transfer torque (stt) mram

ABSTRACT

A multilayered magnetic free layer structure is provided that includes a first magnetic free layer and a second magnetic free layer separated by a non-magnetic layer in which the first magnetic free layer is composed of an ordered magnetic alloy. The ordered magnetic alloy provides a first magnetic free layer that has low moment, but is strongly magnetic. The use of such an ordered magnetic alloy first magnetic free layer in a multilayered magnetic free layer structure substantially reduces the switching current needed to reorient the magnetization of the two magnetic free layers.

BACKGROUND

The present application relates to magnetoresistive random access memory(MRAM). More particularly, the present application relates to a magnetictunnel junction (MTJ) pillar including a multilayered magnetic freelayer structure that improves the performance of spin-transfer torque(STT) MRAM.

MRAM is a non-volatile random access memory technology in which data isstored by magnetic storage elements. These elements are typically formedfrom two ferromagnetic plates, each of which can hold a magnetization,separated by a thin dielectric layer (i.e., a tunnel barrier). One ofthe two plates (i.e., the magnetic reference or pinned layer) is apermanent magnet set to a particular polarity; the other plate's (i.e.,the magnetic free layer's) magnetization can be changed to storeinformation. Such elements may be referred to as a magnetic tunneljunction (MTJ) pillar.

One type of MRAM that can use such a MTJ pillar is STT MRAM. STT MRAMhas the advantages of lower power consumption and better scalabilityover conventional magnetoresistive random access memory which usesmagnetic fields to flip the active elements. In STT MRAM, spin-transfertorque is used to flip (switch) the orientation of the magnetic freelayer. Moreover, spin-transfer torque technology has the potential tomake possible MRAM devices combining low current requirements andreduced cost; however, the amount of current needed to reorient (i.e.,switch) the magnetization is at present too high for most commercialapplications.

In the prior art of spin torque switching, the emphasis has been onlowering the magnetic damping (also called Gilbert damping) of themagnetic free layer. The theory suggests that the switching current isdirectly proportional to the damping; see, for example, J. Z. Sun, Phys.Rev. B 62, 570 (2000). Hence, lower damping makes the free layer switchin lower current, which is desirable since it means a smaller celltransistor can be used.

In the prior art of spin torque switching, the emphasis has also been toincrease the perpendicular magnetic anisotropy field, H_(k), of themagnetic material of each magnetic free layer in order to allow thereduction of the moment of the free layer to enable fast switching whilemaintaining high activation energy, which provides good data retention.In the single domain model, the activation energy, Eb, is proportionalto the product of the anisotropy field and the free layer magneticmoment, m, as in Eb=½ mH_(k). Reducing m reduces the number of spins inthe free layer and therefore, due to conservation of angular momentum,reduces the time required to switch. Therefore, in order to maintain therequired Eb, the prior art requires increasing H_(k). The magneticanisotropy field, H_(k), is the in-plane field required to saturate aperpendicularly magnetized layer in the in-plane-direction.

FIG. 1 illustrates a prior art MTJ pillar that has been developed inorder to reduce the current needed to reorient (i.e., switch) themagnetization of the active elements. The prior art MTJ pillar of FIG. 1includes a multilayered magnetic free layer structure 14 that containstwo magnetic free layers (16 and 20) separated by a non-magnetic layer18. The prior art MTJ pillar of FIG. 1 also includes a magneticreference (or pinned) layer 10, and a tunnel barrier layer 12. Element16 is the first magnetic free layer that forms an interface with thetunnel barrier layer 12, while element 20 is the second magnetic freelayer that is separated from the first magnetic free layer 16 by thenon-magnetic layer 18. In the drawing, the arrow within the magneticreference layer 10 shows a possible orientation of that layer and thedouble headed arrows in the first and second magnetic free layers (16and 20) illustrate that the orientation in those layers can be switched.The non-magnetic layer 18 is thin enough that the two magnetic freelayers (16 and 20) are coupled together magnetically, so that inequilibrium the first and second magnetic free layers 16 and 20 arealways parallel.

One drawback of the prior art MTJ pillar shown in FIG. 1 is that theswitching of the multilayered magnetic free layer structure 14 can betoo slow in comparison to the length of the applied voltage pulse. This‘drag’ in switching of the orientation of the multilayered magnetic freelayer structure 14 of the prior art MTJ pillar of FIG. 1 may result in awrite error.

There is thus a need for providing MTJ pillars for use in STT MRAMtechnology which include an improved multilayered magnetic free layerstructure that substantially reduces the switching current needed toreorient the magnetization of the multilayered magnetic free layerstructure, while improving the switching speed and even reducing writeerrors of the STT MRAM.

SUMMARY

A multilayered magnetic free layer structure is provided that includes afirst magnetic free layer and a second magnetic free layer separated bya non-magnetic layer in which the first magnetic free layer is composedof an ordered magnetic alloy. The ordered magnetic alloy provides afirst magnetic free layer that has low moment (on order of 100 to 500emu/cm³), but is strongly magnetic (i.e., has a magnetic Curietemperature, Tc, on the order of 200° C. or greater). The use of such anordered magnetic alloy first magnetic free layer in a multilayeredmagnetic free layer structure substantially reduces the switchingcurrent needed to reorient the magnetization of the two magnetic freelayers.

In one aspect of the present application, a magnetic tunnel junction(MTJ) pillar is provided. In one embodiment, the MTJ pillar includes amultilayered magnetic free layer structure including a first magneticfree layer and a second magnetic free layer separated by a non-magneticlayer. In accordance with the present application, the first magneticfree layer is composed of an ordered magnetic alloy.

In another aspect of the present application, a spin-transfer torquemagnetic random access memory (STT MRAM) is provided. In one embodiment,the STT MRAM includes a magnetic tunnel junction pillar that is locatedbetween a bottom electrode and a top electrode. The magnetic tunneljunction pillar includes a multilayered magnetic free layer structureincluding a first magnetic free layer and a second magnetic free layerseparated by a non-magnetic layer. In accordance with the presentapplication, the first magnetic free layer is composed of an orderedmagnetic alloy.

In yet another aspect of the present application, a method of improvingthe performance of a spin-transfer torque magnetic random access memory(STT MRAM) is provided. In one embodiment, the method includes providinga multilayered magnetic free layer structure on a surface of a tunnelbarrier layer that is located on a magnetic reference layer. Themultilayered magnetic free layer structure includes a first magneticfree layer and a second magnetic free layer separated by a non-magneticlayer. In accordance with the present application, the first magneticfree layer is composed of an ordered magnetic alloy.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross sectional view of a prior art MTJ pillar including amultilayered magnetic free layer structure which includes two magneticfree layers separated by a non-magnetic layer.

FIG. 2 is a cross sectional view of an exemplary MTJ pillar inaccordance with one embodiment of the present application.

FIG. 3 is a cross sectional view of an exemplary MTJ pillar inaccordance with another embodiment of the present application.

DETAILED DESCRIPTION

The present application will now be described in greater detail byreferring to the following discussion and drawings that accompany thepresent application. It is noted that the drawings of the presentapplication are provided for illustrative purposes only and, as such,the drawings are not drawn to scale. It is also noted that like andcorresponding elements are referred to by like reference numerals.

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to provide an understanding ofthe various embodiments of the present application. However, it will beappreciated by one of ordinary skill in the art that the variousembodiments of the present application may be practiced without thesespecific details. In other instances, well-known structures orprocessing steps have not been described in detail in order to avoidobscuring the present application.

It will be understood that when an element as a layer, region orsubstrate is referred to as being “on” or “over” another element, it canbe directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “beneath” or “under” another element, it can bedirectly beneath or under the other element, or intervening elements maybe present. In contrast, when an element is referred to as being“directly beneath” or “directly under” another element, there are nointervening elements present.

Referring first to FIG. 2, there is illustrated an exemplary MTJ pillarin accordance with one embodiment of the present application. The term“pillar” as used in the present application denotes a multilayeredstructure that has been formed by deposition and photolithography. TheMTJ pillar of FIG. 2 includes a magnetic reference layer 30, a tunnelbarrier layer 32, and a multilayered magnetic free layer structure 34that includes a first magnetic free layer 36, a non-magnetic layer 38,and a second magnetic free layer 40. The MTJ pillar of FIG. 2 istypically present on a bottom electrode (not shown) that is located on asurface of an electrically conductive structure (not shown) that isembedded in an interconnect dielectric material layer (also not shown).A MTJ cap and a top electrode (both not shown) can be formed above atopmost surface of the second magnetic free layer 40. The MTJ pillar ofFIG. 2, together with the MTJ cap and top electrode can be embeddedwithin a second interconnect dielectric material layer.

The bottom electrode may be composed of an electrically conductivematerial and can be formed utilizing techniques well known to thoseskilled in the art. The bottom electrode may be formed on a recessedsurface or a non-recessed surface of the electrically conductivestructure that is embedded in the first interconnect dielectric materiallayer.

The magnetic reference layer 30 has a fixed magnetization and it istypically formed upon on the bottom electrode (not shown). The magneticreference layer 30 may be composed of a metal or metal alloy thatincludes one or more metals exhibiting high spin polarization. Inalternative embodiments, exemplary metals for the formation of magneticreference layer 30 include iron, nickel, cobalt, chromium, boron, andmanganese. Exemplary metal alloys may include the metals exemplified bythe above. In another embodiment, the magnetic reference layer 30 may bea multilayer arrangement having (1) a high spin polarization regionformed from of a metal and/or metal alloy using the metals mentionedabove, and (2) a region constructed of a material or materials thatexhibit strong perpendicular magnetic anisotropy (strong PMA). Exemplarymaterials with strong PMA that may be used include a metal such ascobalt, nickel, platinum, palladium, iridium, or ruthenium, and may bearranged as alternating layers. The strong PMA region may also includealloys that exhibit strong PMA, with exemplary alloys includingcobalt-iron-terbium, cobalt-iron-gadolinium, cobalt-chromium-platinum,cobalt-platinum, cobalt-palladium, iron-platinum, and/or iron-palladium.The alloys may be arranged as alternating layers. In one embodiment,combinations of these materials and regions may also be employed, andsome layers within 30 may be coupled antiparallel to each other in orderto reduce the dipole fringing field on the free layer. The thickness ofmagnetic reference layer 30 will depend on the material selected. In oneexample, magnetic reference layer 30 may have a thickness from 0.3 nm to3 nm. The magnetic reference layer 30 can be formed by utilizing adeposition process such as, for example, chemical vapor deposition(CVD), plasma enhanced chemical vapor deposition (PECVD), physical vapordeposition (PVD), or sputtering.

Tunnel barrier layer 32, which is formed on the magnetic reference layer30, is composed of an insulator material and is formed at such athickness as to provide an appropriate tunneling resistance. Exemplarymaterials for the tunnel barrier layer 32 include magnesium oxide,aluminum oxide, and titanium oxide, or their combinations, or materialsof higher electrical tunnel conductance, such as semiconductors orlow-bandgap insulators. The thickness of the tunnel barrier layer 32will depend on the material selected. In one example, the tunnel barrierlayer 32 may have a thickness from 0.5 nm to 1.5 nm. The tunnel barrierlayer 32 can be formed by utilizing a deposition process such as, forexample, CVD, PECVD, PVD, or sputtering.

The first magnetic free layer 36 of the multilayered magnetic free layerstructure 34 of the present application, which is formed on the tunnelbarrier layer 32, is composed of an ordered magnetic alloy with amagnetization that can be changed in orientation relative to themagnetization orientation of the magnetic reference layer 30. Asmentioned above, the ordered magnetic alloy that provides the firstmagnetic free layer 36 has a low moment (on order of on order of 100 to500 emu/cm³), but is strongly magnetic (i.e., has a magnetic Curietemperature, Tc, on the order of 200° C. or greater). By “orderedmagnetic alloy” it is meant a magnetic alloy that has a latticestructure in which atoms of one element occupy particular sites andatoms of at least one other element occupy other sites.

In one embodiment, the ordered magnetic alloy is a Heusler alloy. Theterm “Heusler alloy” is used herein to denote an intermetallic ternarycompound of the formula X₂YX, which possesses the Heusler ofhalf-Heusler crystal structure. Exemplary Heusler alloys that can beused in the present application include, but are not limited to, Mn₃Ge,Mn₃Ga, Co₂MnSi, Mn₃Sn or Mn₃Sb.

In another embodiment, the ordered magnetic alloy is a L10 alloy. Theterm “L10 alloy” denotes an intermetallic compound with a body centeredtetragonal crystal structure wherein one element occupies the corners ofthe lattice cell and the other element occupies the body center.Exemplary L10 alloys that can be used in the present applicationinclude, but are not limited to, MnAl or CoFe.

The first magnetic free layer 36 can be formed by utilizing a depositionprocess such as, for example, CVD, PECVD, PVD, or sputtering. The firstmagnetic free layer 36 has a first perpendicular magnetic anisotropyfield which can be from 2 kOe to 20 kOe. The first magnetic free layer36 has a first thickness. In one embodiment, the first thickness of thefirst magnetic free layer 36 is from 0.8 nm to 3 nm.

The non-magnetic layer 38 of the multilayered magnetic free layerstructure 34 is composed of a non-magnetic material that contains atleast one element with an atomic number less than 74 such as, forexample, beryllium (Be), magnesium (Mg), aluminum (Al), calcium (Ca),boron (B), carbon (C), silicon (Si), vanadium (V), chromium (Cr),titanium (Ti), manganese (Mn) or any combination including alloysthereof. The thickness of the non-magnetic layer 34 is thin enough toallow the first and second magnetic free layers (36, 40) to coupletogether magnetically so that in equilibrium layers 36 and 40 are alwaysparallel. In one example, the non-magnetic layer 38 has a thickness from0.3 nm to 3.0 nm. The non-magnetic layer 38 can be formed by utilizing adeposition process such as, for example, CVD, PECVD, PVD, or sputtering.

The second magnetic free layer 40 of the multilayered magnetic freelayer structure 34 is formed on the nonmagnetic material layer 38. Thesecond magnetic free layer 40 may include a magnetic material or astacked of magnetic materials with a magnetization that can also bechanged in orientation relative to the magnetization orientation of themagnetic reference layer 30. Exemplary materials for the second magneticfree layer 40 include alloys and/or multilayers of cobalt, iron, alloysof cobalt-iron, nickel, alloys of nickel-iron, and alloys ofcobalt-iron-boron. The second magnetic free layer 40 can be formed byutilizing a deposition process such as, for example, CVD, PECVD, PVD, orsputtering.

The second magnetic free layer 40 has a second perpendicular magneticanisotropy field which can be from 1 kOe to 5 kOe. The second magneticfree layer 40 has a second thickness which is typically, but notnecessarily always, greater than the first thickness of the firstmagnetic free layer 36. In one embodiment, the second thickness of thesecond magnetic free layer 40 is from 1.5 nm to 4 nm.

In some embodiments (not shown), a MTJ cap layer can be formed on aphysically exposed surface of the second magnetic free layer 40. Whenpresent, the MTJ cap may be composed of Nb, NbN, W, WN, Ta, TaN, Ti,TiN, Ru, Mo, Cr, V, Pd, Pt, Rh, Sc, Al or other high melting pointmetals or conductive metal nitrides. The MTJ cap layer may be formedutilizing a deposition process including, for example, CVD, PECVD, ALD,PVD, sputtering, chemical solution deposition or plating. The MTJ caplayer may have a thickness from 2 nm to 25 nm; other thicknesses arepossible and can be used in the present application as the thickness ofthe MTJ cap layer.

The top electrode, which is formed above the second magnetic free layer40, includes an electrically conductive material. In one embodiment, thetop electrode is composed of a same electrically conductive material asthe bottom electrode. In another embodiment, the top electrode may becomposed of a compositionally different electrically conductive materialthan the bottom electrode. The top electrode may be formed utilizing adeposition process. The top electrode is typically formed within anopening that is provided to an interconnect dielectric material layerthat is formed laterally adjacent and atop the MTJ pillar of FIG. 2.

Referring now to FIG. 3, there is illustrated an exemplary MTJ pillar inaccordance with another embodiment of the present application. The MTJpillar of FIG. 3 is similar to the MTJ pillar shown in FIG. 2 exceptthat the MTJ pillar of FIG. 3 includes an interfacial first magneticfree layer 35 located between the tunnel barrier 32 and the firstmagnetic free layer 36 that is composed of the ordered magnetic alloy.Notably, the MTJ pillar of FIG. 3 includes magnetic reference layer 30,tunnel barrier layer 32, and a multilayered magnetic free layerstructure 34 that includes the interfacial first magnetic free layer 35,first magnetic free layer 36, non-magnetic layer 38, and second magneticfree layer 40.

The MTJ pillar of FIG. 3 is typically present on a bottom electrode (notshown) that is located on a surface of an electrically conductivestructure (not shown) that is embedded in an interconnect dielectricmaterial layer (also not shown). A MTJ cap and a top electrode (both notshown) can be formed above a topmost surface of the second magnetic freelayer 40. The MTJ pillar of FIG. 3, together with the MTJ cap and topelectrode can be embedded within a second interconnect dielectricmaterial layer.

In the embodiment illustrated in FIG. 3, the bottom electrode, themagnetic reference layer 30, the tunnel barrier layer 32, the firstmagnetic free layer 36, the non-magnetic layer 38, the second magneticfree layer 40, the MTJ cap, and the top electrode are the same asmentioned above for the exemplary structure shown in FIG. 2.

The interfacial first magnetic free layer 35 is composed of a CoFealloy. The amount of Co and Fe present in the CoFe alloy may vary.Typically, the CoFe alloy contains from greater than 0 atomic percent to50 atomic percent Co, and from 50 atomic percent to less than 100 atomicpercent Fe. The CoFe alloy can be formed by utilizing a depositionprocess such as, for example, CVD, PECVD, PVD, or sputtering. In oneembodiment, the interfacial first magnetic free layer 35 may have athickness from 0.2 nm to 1 nm. The interfacial first magnetic free layer35 is used in order to maintain high magnetoresistance in the MTJpillar.

The multilayered magnetic free layer structure 34 of the presentapplication shown in either FIG. 2 or FIG. 3 requires a substantiallyreduced switching current needed to reorient the magnetization of themagnetic free layers (36, 40). Notably, the use of the ordered magneticalloy as the first magnetic free layer 36 improves the switching speedof the magnetic free layers and thus reduces, and even eliminates, writeerrors.

While the present application has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present application. It is therefore intended that the presentapplication not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

1. A magnetic tunnel junction pillar comprising: a multilayered magneticfree layer structure comprising a first magnetic free layer and a secondmagnetic free layer separated by a non-magnetic layer, wherein the firstmagnetic free layer is composed of ordered magnetic alloy, and thesecond magnetic free layer is composed of at least one magnetic materialthat differs from the ordered magnetic alloy and is selected from thegroup consisting of cobalt, iron, a cobalt-iron alloy, nickel, anickel-iron alloy and a cobalt-iron-boron alloy; a tunnel barrier layerlocated on a surface of the first magnetic free layer opposite a surfaceof the first magnetic free layer that forms an interface with thenon-magnetic layer; and a magnetic reference layer located on a surfaceof the tunnel barrier layer that is opposite the surface of the tunnelbarrier that is located on the first magnetic free layer.
 2. Themagnetic tunnel junction pillar of claim 1, wherein the ordered magneticalloy is a Heusler alloy.
 3. The magnetic tunnel junction pillar ofclaim 2, wherein the Heusler alloy comprises Mn₃Ge, Mn₃Ga, Co₂MnSi,Mn₃Sn or Mn₃Sb.
 4. The magnetic tunnel junction pillar of claim 1,wherein the ordered magnetic alloy is a L10 alloy.
 5. The magnetictunnel junction pillar of claim 4, wherein the L10 alloy comprises MnAlor CoFe.
 6. (canceled)
 7. The magnetic tunnel junction pillar of claim1, further comprising an interfacial first magnetic free layer composedof CoFe alloy located between the first magnetic free layer and thetunnel barrier layer.
 8. A spin-transfer torque magnetic random accessmemory comprising: a magnetic tunnel junction pillar located between abottom electrode and a top electrode, the magnetic tunnel junctionpillar comprising a multilayered magnetic free layer structurecomprising a first magnetic free layer and a second magnetic free layerseparated by a non-magnetic layer, wherein the first magnetic free layeris composed of ordered magnetic alloy, and the second magnetic freelayer is composed of at least one magnetic material that differs fromthe ordered magnetic alloy and is selected from the group consisting ofcobalt, iron, a cobalt-iron alloy, nickel, a nickel-iron alloy and acobalt-iron-boron alloy; a tunnel barrier layer located on a surface ofthe first magnetic free layer opposite a surface of the first magneticfree layer that forms an interface with the non-magnetic layer; and amagnetic reference layer located on a surface of the tunnel barrierlayer that is opposite the surface of the tunnel barrier that is locatedon the first magnetic free layer.
 9. The spin-transfer torque magneticrandom access memory of claim 8, wherein the ordered magnetic alloy is aHeusler alloy.
 10. The spin-transfer torque magnetic random accessmemory of claim 9, wherein the Heusler alloy comprises Mn₃Ge, Mn₃Ga,Co₂MnSi, Mn₃Sn or Mn₃Sb.
 11. The spin-transfer torque magnetic randomaccess memory of claim 8, wherein the ordered magnetic alloy is a L10alloy.
 12. The spin-transfer torque magnetic random access memory ofclaim 11, wherein the L10 alloy comprises MnAl or CoFe.
 13. (canceled)14. The spin-transfer torque magnetic random access memory of claim 8,further comprising an interfacial first magnetic free layer composed ofCoFe alloy located between the first magnetic free layer and the tunnelbarrier layer.
 15. A method of improving the performance ofspin-transfer torque magnetic random access memory, the methodcomprising: providing a multilayered magnetic free layer structure on asurface of a tunnel barrier layer that is located on a magneticreference layer, wherein the multilayered magnetic free layer structurecomprises a first magnetic free layer and a second magnetic free layerseparated by a non-magnetic layer, wherein the first magnetic free layeris located on the tunnel barrier layer and is composed of orderedmagnetic alloy, and the second magnetic is composed of at least onemagnetic material that differs from the ordered magnetic alloy and isselected from the group consisting of cobalt, iron, a cobalt-iron alloy,nickel, a nickel-iron alloy and a cobalt-iron-boron alloy.
 16. Themethod of claim 15, wherein the ordered magnetic alloy is a Heusleralloy.
 17. The method of claim 16, wherein the Heusler alloy comprisesMn₃Ge, Mn₃Ga, Co₂MnSi, Mn₃Sn or Mn₃Sb.
 18. The method of claim 15,wherein the ordered magnetic alloy is a L10 alloy.
 19. The method ofclaim 15, wherein the L10 alloy comprises MnAl or CoFe.
 20. The methodof claim 15, further comprising forming an interfacial first magneticfree layer composed of CoFe alloy between the first magnetic free layerand the tunnel barrier layer.